The present invention relates to a method for fabricating photomasks used for the production of high-density integrated circuits such as LSIs, VLSIs, etc., and more particularly to a method for fabricating a halftone phase shift photomask used for forming fine patterns with high accuracy.
Semiconductor integrated circuits such as ICs, LSIs and VLSIs are now fabricated by repeating the so-called lithography process wherein a resist is coated on the substrate to be processed, like a silicon wafer, and exposed to a desired pattern through a stepper, etc., followed by development, etching, doping, CVD, etc.
A photomask used for such a lithography process and called a reticle is now increasingly required to have much higher accuracy in association with the high performance and high integration of semiconductor integrated circuits. Referring to a typical LSI, i.e., a DRAM by way of example, a 5.times. reticle for a 1 megabit DRAM, i.e., a reticle of size five times as large as that of an exposure pattern should be very small in terms of dimensional variation; the accuracy demanded is as small as 0.15 .mu.m even at the mean value .+-.3.sigma. (.sigma. is the standard deviation). Likewise, a dimensional accuracy of 0.1 to 0.15 .mu.m is demanded for 5.times. reticles for 4 megabit DRAMs; and a dimensional accuracy of 0.05 to 0.1 .mu.m for 5.times. reticles for 16 megabit DRAMs.
In addition, the line widths of device patterns formed with the use of these reticles are now becoming finer; for instance, they must be 1.2 .mu.m for 1 megabit DRAMs, 0.8 .mu.m for 4 megabit DRAMs, and 0.6 .mu.m for 16 megabit DRAMs. To meet such demands, various photolithography technologies are now under investigation.
In the case of the next generation device patterns of the 64 megabit DRAM class for instance, however, the use of stepper photolithography technologies using conventional reticles will place some limit on resolving the resist patterns. To exceed this limit, a reticle known as a phase shift mask and designed on the basis of a new technological paradigm has been proposed in the art, as set forth in JP-A-58-173744, JP-B-62-59296, etc. Phase lithography making use of this phase shift reticle is a technology that enables the resolving power and contrast of a projected image to be increased by manipulation of the phase of light transmitting through the reticle.
A halftone phase shift photomask is now proposed as one of such phase shift masks. This halftone phase shift photomask will now be explained briefly with reference to FIGS. 3(a-d) and 4(a-d). FIG. 3(a-d) is a schematic of the principle of halftone phase shift lithography, and FIG. 4(a-d) is a schematic of a conventional process. FIGS. 3(a) and 4(a) are sectional views of the reticles used, FIGS. 3(b) and 4(b) represent the amplitude of light transmitting through the reticles, FIGS. 3(c) and 4(c) illustrate the amplitude of light on the wafers, and FIGS. 3(d) and 4(d) show the intensity of light on the wafers. Reference numeral 1 is a substrate, 2 a 100% light-blocking layer, 3 a halftone light-blocking film having a transmittance of 5 to 30%, 4 a phase shift layer, and 5 incident light. In the conventional arrangement, the substrate 1 made up of glass or other material is simply provided with the 100% light-blocking layer 2 for the purpose of defining a light transmitting portion according to a given pattern, as shown in FIG. 4(a). In the halftone phase shift lithographic arrangement, however, the halftone light-blocking film 3 is provided thereon with the phase shift layer 4 made up of a light-transmitting film for phase reversal (with a 180.degree. phase difference), as shown in FIG. 3(a). In the conventional process, therefore, the amplitude of light on the reticle is in the same phase, as shown in FIG. 4(b), and so is the amplitude of light on the wafer, as shown in FIG. 4(c); the light intensity distribution on the wafer does not take the form corresponding to the mask pattern, and spreads outwardly from the portions under the opening in the mask, as can be seen from FIG. 4(d). In the case of halftone phase shift lithography, in contrast, the light passing through the halftone light-blocking film 3 and the phase shift layer 4 is in the opposite phase with respect to the light passing through the openings in the mask, as can be seen from FIG. 3(b), so that the intensity of light can be reduced to zero at the pattern boundary, thus making it possible to prevent the light intensity distribution from spreading outwardly, as can be seen from FIG. 4(d). Thus, halftone phase shift lithography makes it possible to resolve a pattern which, until now, cannot be resolved, resulting in an improvement in resolution.
Then, reference will be made to how to fabricate a typical halftone phase shift photomask so far brought forward in the art. FIG. 5 is a sectional schematic showing the photosteps of fabricating it, in which 19 stands for a substrate, 20 an electrically conductive layer, 21 a halftone light-blocking layer, 22 a phase shift layer, 23 a resist, 24 a resist pattern, 25 an etching gas, 26 a halftone light-blocking pattern, and 27 a phase shifter pattern.
First, a halftone phase shift photomask blank is prepared by a conventional procedure such as one shown in FIG. 5(a), followed by pattern defect inspection. Then, as shown in FIG. 5(b), the ionizing radiation resist 23 made up of, e.g., chloromethylated polystyrene is uniformly coated on the blank by spin coating or other known techniques, followed by drying-by-heating. This drying-by-heating treatment may usually be done at 80.degree. to 150.degree. C. for 20 to 60 minutes, although varying depending on the type of resist used. Then, a predetermined pattern is written on the resist layer 23 by means of ionizing radiation in conventional manners with an exposure system such as an electron beam lithography system, developed with a developer composed mainly of an organic solvent such as ethyl cellosolve or ester, and rinsed with alcohol to form such a resist pattern as shown in FIG. 5(c).
If needed, heat and descumming treatments are subsequently conducted for removal of undesired matters such as resist scum and whisker which remain on the edge portions of the resist pattern 24. Following this, as shown in FIG. 5(d), the portions of the phase shift layer 22 and the halftone light-blocking layer 21 under the openings in the resist pattern 24 are continuously dry etched, while the conditions for the etching gas plasma 25 are varied, thereby forming the halftone light-blocking pattern 26 and phase shifter pattern 27. As will be obvious to those skilled in the art, both the patterns 26 and 27 may also be formed by use of wet etching instead of dry etching for which the etching gas plasma 25 is used.
After this, the resist pattern 24, i.e., the remaining resist is stripped off using a solvent to obtain a photomask. By washing and inspecting this photomask, a halftone phase shift photomask having the phase shifter pattern 27 is completed, as shown in FIG. 5(e).
For the above-mentioned conventional halftone phase shift photomask, however, it is necessary to provide two layers, i.e., the phase shift and halftone light-blocking layers, resulting in an increase in the number of the steps of blank preparation. There is also an increase in the number of the steps of mask fabrication by reason of a difference in the conditions for etching the halftone light-blocking and phase shift layers. Moreover, it is required to conduct defect inspection for each layer. These pose many problems, for instance, considerable cost, much time needed for mask preparation, the frequent occurrence of defects, etc.